System for converting charge into voltage and method for controlling this system

ABSTRACT

The invention relates to controlling a device for converting charge into voltage comprising an amplifier and at least one capacitor mounted in inverse feedback between an input and an output of said amplifier, whereby said amplifier can be connected between at least one input stage, to receive a charge therefrom, and at least one output stage to deliver voltage thereto, said voltage being representative of the charge received at the input, said method comprising at least one phase comprising the voltage conversion of a charge received at the input. According to the invention the conversion phase comprises at least: one first sub-phase during which the amplifier is connected to the input stage and the amplifier is disconnected from the output stage; followed, by a second sub-phase during which the amplifier is disconnected from the input stage and the amplifier is connected to the output stage.

FIELD OF THE INVENTION

The invention relates to the charge conversion field.

To be more specific, the invention relates to a method for controlling adevice for converting charges comprising an amplifier and at least onecapacitor mounted in inverse feedback between an input and an output ofsaid amplifier, whereby said amplifier can be connected between at leastone input stage, to receive a charge therefrom, and at least one outputstage to deliver voltage thereto, said voltage being representative ofthe input charge, said method including at least one phase comprisingthe voltage conversion of the charge received at the input.

BACKGROUND OF THE INVENTION

Charge amplification by means of an operational amplifier with inversefeedback via a capacitor is used in many readout applications, such asfor example voltage readout, capacitor readout or charge packet readout.

As is known per se, an operational amplifier has a limited voltage gainin the defined range of its supply voltages. The gain is further limitedto a range of frequencies, with the amplifier usually having a gain ofthe low-pass type. Moreover, this frequency range depends on theimpedance of the charges connected at the amplifier input and output,charges which therefore have an effect on the time taken to establishthe amplified signal at the amplifier output.

To overcome these drawbacks, an impedance is conventionally mounted ininverse feedback between the inverting input and the output of theamplifier, in such a way that the transfer function of the system solooped does not depend predominantly on this impedance.

When the inverse feedback impedance is a capacitor, we then talk aboutcharge amplification. Indeed, an operational amplifier with inversefeedback via a capacitor amplifies the charges it receives at input.

FIG. 1 shows a prior art device for converting charge into voltage 10.This amplification device 10 is connected at input to an input stage 12and at output to an output stage 14.

The conversion device 10 includes an operational amplifier 16, oftransconductance g_(m), and an inverse feedback capacitor 18, of valueC_(fb), connected between the inverting terminal 20 and the outputterminal 22 thereof. The non-inverting terminal 24 of the amplifier 16is for its part connected to a reference potential, such as an earth 26for example.

The input stage 12 comprises a voltage source 28, connected between theearth 26 and a terminal 30 and of value V_(e), and a voltage conversioncapacitor 32, connected between the terminals 30 and 31, of valueC_(mes).

The output stage 14 comprises for its part an input equivalent capacitor34 of value C_(out) and connected between the output terminal 22 of theamplifier 16 and the earth 26.

In the diagram in FIG. 1, parasitic capacitors 36, 38, 40 are alsoshown. These capacitors are generated by the structure of thetransistors, of MOS technology for example, that constitute theamplifier 16, and by the various interconnections (tracks, wires, metalsolders, etc.) which exist in, and between, the different elements thathave just been described.

Conventionally, these parasitic capacitors 36, 38, 40 are modelled by:

an input parasitic capacitor 36 of the amplifier 16 of value C_(pin).This parasitic capacitor 36 is connected between the inverting terminal20 thereof and the earth 26;

an output parasitic capacitor 38 of the amplifier 16 of value C_(pout).This parasitic capacitor 38 is connected between the output terminalthereof and the earth 26;

and a parasitic capacitor 40 of the input stage connections, of valueC_(is). This parasitic capacitor 40 is connected in parallel to theinput parasitic capacitor 36 of the amplifier.

Controllable circuit breakers 42, 44, 46, 48, 50, 52 are also provided,a first circuit breaker 42 being connected in parallel to the inversefeedback capacitor 18, a second circuit breaker 44 being connected atthe input of the amplifier 16 between the inverting terminal 20 and theterminal 31 of the capacitor 32 of the input stage 12, a third circuitbreaker 46 being connected between this capacitor 32 and the voltagesource 28, a fourth circuit breaker 48 being connected between theterminal 30 of the input stage and the earth 26, a fifth circuit breaker50 being connected between the terminal 31 of the input stage and theearth 26, and a sixth circuit breaker 52 being connected between theoutput 22 of the amplifier 16 and the input equivalent capacitor 34 ofthe output stage 14.

The circuit breakers 42, 44, 46, 48, 50, 52 are controlled by agenerator 70 of two control signals φ₁, φ₂ in accordance with a strategyfor switching on and off as described hereinafter, the signal φ₁controlling the switching on and off of the circuit breakers 42, 48 and50, and the signal φ₂ controlling the switching on and off of thecircuit breakers 44, 46 and 52.

Depending on the nature of the circuit breakers, the generator 70 maypossibly be led to deliver signals that are complementary in terms ofbinary logic, particularly in respect of controlling CMOS circuitbreakers.

FIG. 1 for example shows amplification of the voltage V_(e) by means ofcharge amplification.

Initially, the first, fourth and fifth circuit breakers 42, 48 and 50are off and the second, third and sixth circuit breakers 44, 46, 52 areon. The inverse feedback capacitors 18 and voltage conversion capacitors32 are therefore discharged.

Secondly, the first, fourth and fifth circuit breakers 42, 48 and 50 areon and the second, third and sixth circuit breakers 44, 46, 52 are off.The voltage V_(e) at the terminals of the source 28 is thus convertedinto a charge Q_(e) by the voltage conversion capacitor 32, and anequivalent charge Q_(e)′ (=Q_(e)) is generated at the terminals of theinverse feedback capacitor 18 by conserving the charge at thenon-inverting input 20 of the amplifier 16. This charge Q_(e)′ isconverted into voltage V_(out) by means of the inverse feedbackcapacitor 18. This voltage V_(out) can be observed at the terminals ofthe capacitors C_(pout) 38 and Q_(out) 34.

The transfer function between amplified voltage V_(out) and the voltageV_(e) is thus given by the following formula:

$\begin{matrix}{{G(s)} = \frac{C_{mes}{R_{out}\left( {{- g_{m}} + {C_{fb}s}} \right)}}{\begin{matrix}{{\left( {C_{mes} + C_{is} + C_{pin}} \right)\left( {1 + {\left( {C_{out} + C_{pout}} \right)R_{out}s}} \right)} +} \\{C_{fb}\left( {1 + {{R_{out}\left( {C_{mes} + C_{out} + C_{pout} + C_{pin} + C_{is}} \right)}s}} \right)}\end{matrix}}} & (1)\end{matrix}$where s is the Laplace variable and R_(out) the output impedance of theoperational amplifier 16.

The transfer function G(s) is therefore of the first order low-passtype.

The continuous gain G₀ of the function G(S) is given by the formula:

$\begin{matrix}{G_{0} = {- \frac{C_{mes}g_{m}R_{out}}{C_{mes} + C_{pin} + C_{is} + {C_{fb}\left( {1 + {g_{m}R_{out}}} \right)}}}} & (2)\end{matrix}$

The cutoff frequency ω_(c) is furthermore equal to:

$\begin{matrix}{\omega_{c} = \frac{g_{m}}{C_{eq}}} & (3)\end{matrix}$where C_(eq) is a capacitor according to the formula:

$\begin{matrix}{C_{eq} = \frac{\begin{matrix}{{\left( {C_{pout} + C_{out}} \right)\left( {C_{mes} + C_{pin} + C_{is}} \right)} +} \\{C_{fb}\left( {C_{pout} + C_{out} + C_{mes} + C_{pin} + C_{is}} \right)}\end{matrix}}{C_{fb}}} & (4)\end{matrix}$

It will be noted that the capacitor C_(eq) may be rewritten according tothe formula:

$\begin{matrix}{C_{eq} = \frac{{C_{S}C_{E}} + {C_{fb}\left( {C_{S} + C_{E}} \right)}}{C_{fb}}} & (5)\end{matrix}$where C_(E)=C_(mes)+C_(pin)+C_(is) is the capacitor seen at input by theamplifier 16 and C_(S)=C_(pout)+C_(out) is the capacitor seen at outputby the amplifier 16.

It can thus be seen from a consideration of the formulae (2) and (3)that to increase the amplification passband and therefore the speedthereof (the higher the cutoff pulsatance ω_(c) the shorter theamplifier output signal establishment time), ω_(c) needs to bemaximized.

To do this, it is possible to minimize the equivalent capacitance C_(eq)or to maximize the transconductance g_(m) of the amplifier 16.

However, increasing the transconductance g_(m) involves using ahigh-energy consuming amplifier. Additionally, increasing thetransconductance g_(m) also results, in respect of the amplifier, in atransistor geometry with larger parasitic capacitors C_(pin) andC_(pout). The equivalent capacitance C_(eq) is then larger and thecutoff pulsatance ω_(c) smaller.

The passband may also be increased by minimizing the equivalentcapacitance C_(eq).

To lower the value thereof, it is possible to maximize the value of theinverse feedback capacitor C_(ƒb). In fact, maximizing C_(ƒb) has theeffect of reducing the continuous gain G₀, which runs counter to theprimary intended aim, namely amplification.

That is why, passband maximization is usually sought and obtained byminimizing the parasitic capacitors C_(pin) and C_(pout). Research hasthus been carried out on the structure and geometry of the transistorsconstituting the amplifier 16. Said research is however long and complexin so far as it relates to transistor design.

Furthermore, even though the parasitic capacitors might be optimized,the equivalent capacitance C_(eq) still depends on the input 12 andoutput 14 stages, and particularly on the capacitors C_(mes), C_(is) andC_(out). The passband gain, and therefore the establishment time gain,is limited by the presence of these capacitors. Likewise, there arestill parasitic capacitors at the amplifier input and output which arenot connected to the amplifier itself. The parasitic capacitors of theconnections of the amplifier 16 with the input 12 and output 14 stagesmay be cited in particular.

It will be noted that the problems disclosed above are posed in the sameway in other charge amplification applications. For example, it willalso be noted that FIG. 1 shows the readout of the value C_(mes) of thecapacitor 32 whereof the value is unknown.

In such an application the value of the voltage V_(e) is known and thevoltage V_(out) at the terminals of the charge conversion capacitor 34measured. The transfer function between the voltage V_(out) and thecapacitor C_(mes) is then given by the formula:

$\begin{matrix}{G_{0} = \frac{V_{e}g_{m}R_{out}}{C_{mes} + C_{pin} + C_{is} + {C_{fb}\left( {1 + {g_{m}R_{out}}} \right)}}} & \left( {2C} \right)\end{matrix}$

This transfer function is also of the lowpass type with cutoffpulsatance similar to that in the formula (3). Said application is forexample described in the document by N. Yazdi et al. “Precision readoutcircuits for capacitive microaccelerometers”, Sensors 2004, Proceedingsof IEEE.

It will be noted that said application generally comprises measuring thevariations in the voltage conversion capacitor 32 (C_(mes)) around areference value that is much higher than said variations. This involvesin particular choosing the inverse feedback capacitor 18 to be of thesame order of magnitude as the variations in the capacitor 32, so thatthe equivalent capacitance is substantially equal to

$C_{eq} = {\frac{C_{S}C_{E}}{C_{fb}}.}$The influence of the parasitic capacitors on the amplification passbandis therefore strengthened as a result.

These problems are also posed in the case of voltage conversion ofcharge packets from a plurality of input stages sharing a singleconversion device. The input stages are for example the pixels of acolumn of a matrix sensor such as a CCD or CMOS image sensor, whichperiodically delivers charge packets for conversion into voltage on acolumn bus.

In a similar way to the aforementioned applications, the transferfunction between the voltage V_(out) and a received charge packet Q_(e)is of the low-pass type according to the formula:

$\begin{matrix}{G_{0} = \frac{g_{m}R_{out}}{C_{mes} + C_{pin} + C_{is} + {C_{fb}\left( {1 + {g_{m}R_{out}}} \right)}}} & \left( {2Q} \right)\end{matrix}$

The cutoff pulsatance of this transfer function is therefore similar tothat of the formula (3). It will be noted that in this application, theparasitic capacitor connected to the column bus connection is verylarge, thereby limiting the voltage conversion passband of the incidentcharge packets.

The aim of the invention is to resolve the abovementioned problem byproposing a method for controlling the conversion device which allows asignificant gain in passband, and therefore in establishment time, anddoes so by modifying at least the structure, the operation or thearrangement of the amplifier or of the input and output stages, andwithout getting a reduction in the final conversion gain.

SUMMARY OF THE INVENTION

To this end, the purpose of the invention is a method for controlling acharge conversion device comprising an amplifier and at least onecapacitor mounted in inverse feedback between an input and an output ofsaid amplifier, whereby said amplifier can be connected between at leastone input stage, to receive a charge therefrom, and at least one outputstage to deliver voltage thereto, said voltage being representative ofthe incident charge received, said method comprising at least one phasecomprising the voltage conversion of the charge received at the input.

According to the invention, the conversion phase comprises at least:

a first sub-phase during which the amplifier is connected to the inputstage and the amplifier is disconnected from the output stage;

followed, by a second sub-phase during which the amplifier isdisconnected from the input stage and the amplifier is connected to theoutput stage.

In other words, the amplifier is isolated in turn from the input stageand the output stage capacitors. At each of the sub-phases, theequivalent capacitance so obtained is less than the equivalentcapacitance C_(eq). The accumulated establishment time of these twosub-phases is thus less than that commonly observed when implementingthe prior art amplification phase.

According to particular embodiments of the invention, the methodcomprises one or more of the following characteristics:

the durations of two sub-phases are substantially identical;

the durations of two sub-phases are regulated as a function of the timeconstants of the units formed, on the one hand, by the amplificationdevice connected to the input stage, and on the other hand, by theamplification device connected to the output stage;

the or each amplifier is an operational amplifier whereof the invertinginput and the output are mounted in inverse feedback via a capacitor;

the or each amplifier is a differential operational amplifier, whereofeach input is mounted in inverse feedback with an output of theamplifier by means of a capacitor;

A further purpose of the invention is a charge amplification system.This includes:

an amplification device including an amplifier and at least onecapacitor mounted in inverse feedback between an input and an output ofsaid amplifier, whereby said amplifier can be connected between at leastone input stage, to receive a charge therefrom, and at least one outputstage to deliver voltage thereto, said voltage being representative ofthe charge received at the input;

a first controllable element capable of discharging the inverse feedbackcapacitor,

a second controllable element capable of disconnecting the amplifierfrom the input stage;

a controllable device capable of controlling the first and secondcontrollable elements according to a phase of discharging the inversefeedback capacitor, and a phase of amplifying a charge received from theinput stage by the amplifier.

According to the invention:

the location of the connection and disconnection of the amplifier to andfrom the input stage and/or the location of the connection anddisconnection of the amplifier to and from the output stage is placed asclose as possible to the amplifier;

the system further comprises a third controllable element capable ofdisconnecting the amplifier from the output stage, the control devicebeing capable, at the amplification phase, at least of:

-   -   controlling the second controllable element in order to connect        the amplifier to the input stage and controlling the third        controllable element in order to disconnect the amplifier from        the output stage;    -   then, controlling the second controllable element in order to        disconnect the amplifier from the input stage and controlling        the third controllable element in order to connect the amplifier        to the output stage.

In other words, this system implements the aforementioned method.

According to one embodiment, the input stage is of theMicro-Electro-Mechanical Systems (MEMS) or Nano Electro-MechanicalSystems (NEMS) type and comprises a time-variable capacitor, the inputstage producing a charge as a function of said variable capacitor.

In other words, the invention can be used to advantage to measurevariations in the capacitance of a capacitor incorporated into a MEMS orNEMS device, such as for example an accelerometer or a gyrometer withthe capacitance of the capacitor thereof varying as a function of theaccelerations sustained.

According to one particular embodiment of the invention, the input stageincludes a column of a matrix of detection elements, whereby the chargeconversion device can be connected to each of the detection elements ofsaid column to receive a charge therefrom.

In other words, prior art imaging matrix readout circuits commonlyinclude charge conversion devices at the end of each column of thematrix in order to convert sequentially the electrical charges producedby the unitary detection elements, or pixels, of the column. In fact,prior art charge conversion devices remain constantly connected to thecolumn buses through which the charges from the pixels pass. Said columnbuses commonly have very high parasitic capacitance and the larger thematrix dimensions the bigger this is. This results in a very significantreadout time for the charges produced on account of a high equivalentcapacitance. The effect of disconnecting the charge devices from thesecolumn buses when establishing the voltages at output from theconversion device is thus a significant increase in establishment speed.

By retaining the duration of a prior art readout frame, a conversion andtotal transfer of the charge to the output stage can then be seen.

According to one embodiment of the invention, the first controllableelement and/or the second controllable element is placed as close aspossible to the amplifier.

In other words, the maximum number of elements are connected to anddisconnected from the amplifier, and particularly the connectionsconnecting the amplifier to the input stage and to the output stage.Indeed it is known that these connections commonly have a largeparasitic capacitance. By ensuring connection and disconnection as closeas possible to the physical elements actually responsible for theamplification, the equivalent capacitance is then optimized.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reading the followingdescription, given solely by way of example, and drawn up in relation tothe appended drawings wherein identical reference numbers denoteidentical or similar elements, and wherein:

FIG. 1 is a diagrammatic view of a device for switched-capacitor chargeconversion as in the prior art and as already described in thepre-characterizing portion;

FIG. 2 is a diagrammatic view of a device for switched-capacitor chargeconversion according to a first embodiment of the invention;

FIG. 3 is a flowchart of a method according to the invention;

FIGS. 4 to 7 are curve graphs showing the various charge amplificationphases according to the invention and the prior art in FIG. 1.

FIG. 8 is a diagrammatic view of a prior art charge converter associatedwith a matrix of pixels arranged in L lines and C columns;

FIG. 9 is a flowchart of a method for controlling a charge conversiondevice forming part of the charge converter in FIG. 8;

FIGS. 10 to 17 are timing diagrams for circuit breaker control signalsforming part of the charge converter in FIG. 8;

FIG. 18 is a diagrammatic view of a charge converter according to theinvention associated with a matrix of pixels arranged in L lines and Ccolumns.

FIG. 19 is a flowchart of a method for controlling a charge conversiondevice forming part of the charge converter in FIG. 18;

FIGS. 20 to 28 are timing diagrams for circuit breaker control signalsforming part of the charge converter of the invention in FIG. 18;

FIGS. 29 to 34 are curve graphs showing the various charge amplificationphases using a charge converter of the invention and a prior art chargeconverter; and

FIG. 35 is a diagrammatic view of a differential device forswitched-capacitor charge conversion according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 2, a charge conversion device 60 according to the invention isconnected at input to the input stage 12 and at output to the outputstage 14. The arrangement in FIG. 2 shows for example the amplificationof the voltage V_(e) or the measurement of variations in the voltageconversion capacitor 32, as has been explained in further detail above.

In one advantageous application of the invention, the input stage 12 isa micro-electro-mechanical (or MEMS) device or a nano electro-mechanical(or NEMS) device. For example, the input stage 12 is formed of anaccelerometer or a gyrometer whereof the capacitor 32, formed ofinterdigited combs whereof at least one is mobile, varies in time. Thecharge conversion device 60 is to advantage made using the same MEMS orNEMS technology as the input stage 12.

The charge conversion device 60 differs from the prior art device 10,described hereinafter in relation to FIG. 1, in that it is associatedwith a generator 80 of at least three different control signals φ₁, φ₂₁,φ₂₂ of the circuit breakers 42, 44, 46, 48, 50 and 52, the signal φ₁controlling the switching on and off of the circuit breakers 42, 48 and50, the signal φ₂₁ controlling the switching on and off of the circuitbreakers 44 and 46, and the signal φ₂₂ controlling the switching on andoff of the circuit breaker 52.

Depending on the nature of the circuit breakers the generator 80 willpossibly be led to deliver signals that are complementary in terms ofbinary logic, particularly in respect of CMOS circuit breaker control.

The method for controlling the circuit breakers 42, 44, 46, 48, 50 and52 implemented by the generator 80 is shown in the flowchart in FIG. 3.

During a first step 61, similar to that in the prior art, the first,third and fifth circuit breakers 42, 48, 50 are off and the second,fourth and sixth circuit breakers 44, 46, 52 are on. All the capacitors18, 32, 36, 38, 40 are thus discharged.

The discharge phase of step 61 is then followed, at 62, by a chargeconversion phase.

In a first sub-phase 64 of the conversion phase 62, the first, third andfifth circuit breakers 42, 48, 50 are on, and the second and fourthcircuit breakers 44, 46 are off. The fifth circuit breaker 52 is lefton, the output terminal 22 of the amplifier 16 being therebydisconnected from the output stage 14.

Thus, the voltage V_(e) is converted into a charge Q_(e) by the voltageconversion capacitor 32, and the charge Q_(e) is converted by theamplifier with inverse feedback 16, 18. The converted charge is storedin the inverse feedback capacitor 18, thus generating the voltageV_(out) at the output terminal 22.

In a second sub-phase 66, the second and fourth circuit breakers 44, 46are on, thereby disconnecting the non-inverting input 20 of theamplifier 16 from the input stage 12. The fifth circuit breaker 52 forits part is off, thereby connecting the output of the amplifier 16 tothe output stage 14.

The voltage V_(out) generated at the terminals of the inverse feedbackcapacitor 18 is thus transferred to the output stage 14, in other wordsstored in this instance in the capacitor C_(out) 34 thereof.

The conversion step 62 then re-loops to the step 61 for a new cycle ofamplification of the voltage V_(e) or of readout, in the form ofvoltage, of the capacitor C_(mes) 32.

The three steps 61, 64 and 66 implemented by the generator 80 never havean instant in common. These three steps are always intersected by asmall time fraction during which all the circuit breakers are on so thatthe various charge transfers can be rigorously managed, in other wordswithout loss.

FIGS. 4 to 7 show readout cycles of the voltage conversion capacitor 32,according to the prior art and according to the invention. The capacitor32 is for example that of the MEMS sensor as described in theaforementioned document by Yadzi.

The conventional characteristics of such a MEMS are 1 picofarad for thevalue C_(mes) of the capacitor 32, 5 picofarads for the value C_(is) ofthe parasitic capacitor 40, and the variations ΔC_(mes) of the valueC_(mes) that are required to be measured are of the order of 100femtofarads.

The characteristics of the conversion device, described for example inthe document by Yadzi, are 100 femtofarads for the value C_(pin) of theparasitic capacitor 36, 100 femtofarads for the value C_(fb) of theinverse feedback capacitor 18, 300 μA/V for the value g_(m) of thetransconductance of the amplifier 16 and 1 picofarad for the valueC_(out) of the capacitor 34 and 100 femtofarads for the value C_(pout)of the capacitor 38.

With these values, the equivalent capacitance C_(eq) is then equal to73.6 picofarads, which defines an establishment of 87% of the voltageV_(out) when the prior art phases φ₁ and φ₂ are each equal to 500 ns,i.e. a sampling frequency of 1 MHz for the readout of the variationsΔC_(mes)

The sub-phases 64 and 66 according to the invention are for their partchosen in this instance to be of equal duration, in other words aduration of 250 nanoseconds each.

FIG. 4 shows in respect of both the prior art and the invention, thesignal φ₁ for controlling the circuit breakers 42, 48 and 50. Thecontrol signal value of 2.5 volts equates in this instance to an orderof switching off the corresponding circuit breakers.

FIG. 5 shows, for the prior art arrangement in FIG. 1, the controlsignal φ₂ for the circuit breakers 44, 46 and 52.

FIG. 6 shows, for the arrangement of the invention in FIG. 2, thecontrol signal φ₂₁ for the circuit breakers 44 and 46 and the controlsignal φ₂₂ for the circuit breaker 52.

FIG. 7 shows the voltage at the output terminal 22 of the amplifieraccording to the prior art (thin line curve) and according to theinvention (bold continuous line). As may be noted in FIG. 7, in theprior art, the time for establishing the voltage V_(out) is too long forthe read charge Qe in its entirety to be converted and simultaneouslyavailable on the capacitor 34 of the output stage 14. At the end of theconversion phase, (switching the signal φ₂ from the 2.5 volt value tothe 0 volt value), only 87% of the charge has been transferred to thecapacitor 34 of the output stage 14.

As can be seen, according to the invention, the charge in its entiretyhas been transferred at the end of the amplification phase 62.

Indeed, at the first sub-phase 64, the amplifier 16 is disconnected fromthe output stage 14, and therefore from its capacitor 34. During thisfirst sub-phase 64, the equivalent capacitance C_(eq) is substantiallyequal to 12 picofarads, which gives a time constant of about 40nanoseconds. The converted charge in its entirety is therefore properlystored in the capacitor 18, given that the first sub-phase 64 lasts for250 nanoseconds, this being sufficient time to obtain complete storage.

At the second sub-phase 66, the amplifier 16 is disconnected from theinput stage 12 and therefore from its capacitors 32 and 40. During thissecond sub-phase 66, the equivalent capacitance C_(eq) is substantiallyequal to 2.3 picofarads, which gives a time constant of about 7.7nanoseconds, the voltage V_(out) (representative of the quantity ofcharges converted at the terminals of the inverse feedback capacitor 18)is thus applied at the terminals of the capacitor 34 of the output stage14, given the 250 nanosecond duration of the sub-phase 66.

As may be noted, the accumulated establishment time according to theinvention is about 46.5 nanoseconds. In comparison with the 250nanosecond establishment time of the prior art, a gain above 5 isobtained in the charge conversion speed.

This allows in particular the value g_(m) of the transconductance of theamplifier 16 to be lowered significantly, thereby allowing a substantialsaving of energy. This also allows amplification cycles to be designed,for example readout cycles of the capacitor 32 with a higher frequency.

It will also be noted that this speed gain is achieved without modifyingthe structure of the conversion device which remains identical to thatof the prior art.

The durations of the two phases 64 and 66 making up the conversion phase62 are chosen to be equal so as to have a straightforward control of thecircuit breakers. Given the speed gain obtained, the device of theinvention may thus be incorporated into existing structures with noadditional research.

As an alternative, the durations of the two phases 64 and 66 are chosenas a function of the values of the equivalent capacitance C_(eq) at thetime thereof. For example the durations are chosen in proportion tothese values.

It will be noted that the effect of choosing and sequencing the controlsignals applied to the circuit breakers 44, 46, 48 and 50 is that theinput stage 12 is non-inverting. Since the operational amplifier 16 withinverse feedback via the capacitor 18 is an inverting structure, theassociation of the input stage 12 and the conversion device 60 is then astructure for the amplification of the voltage V_(e) or the readout ofthe capacitor C_(mes) which is inverting, as shown by the “−” sign inthe formula (2).

As an alternative, the signals controlling the circuit breakers 46 and48 previously described are swapped, the circuit breaker 46 then beingcontrolled by the control signal φ₁ and the circuit breaker 48 by thecontrol circuit breaker φ₂₁. The input stage 12 is then inverting.

Since the operational amplifier 16 with inverse feedback via thecapacitor 18 is an inverting structure, the continuous gain is thengiven by formula (2) without the “−” sign. The association of the inputstage 12 and the conversion device 60 is thus a structure foramplification of the voltage V_(e) or readout of the capacitor C_(mes)which is non-inverting.

As an alternative, the circuit breaker 46 (or the circuit breaker 48 inthe “inverting” version of the stage 12 described above) found in theinput stage 12 is controlled by a control signal similar to the signalφ₂ described in conjunction with the prior art device in FIG. 1, insteadof being controlled by the control signal φ₂₁ which controls theconnection/disconnection of the conversion device 60 of the input stage12.

As an alternative, in one or other of the aforementioned input stages(inverting or non-inverting), the circuit breaker 42 of the chargeconversion device 60 is omitted in order to add into this same device 60the various conversions of the input stimuli (V_(e) or C_(mes)) sampledin the input stage 12. The integration function is thus obtained inaddition to the amplification function.

To advantage, the circuit breaker 44 is placed as close as possible tothe node to which the capacitor 18 and the inverting terminal 20 of theoperational amplifier 16 are connected. Switching on the circuit breaker44 thus disconnects the maximum number of connections of those elementsactually responsible for the charge amplification.

For the same reasons, the circuit breaker 52 is placed as close aspossible to the node to which the capacitor 18 and the output terminalof the operational amplifier 16 are connected.

The equivalent capacitances in each connection/disconnection phaseaccording to the invention are thus optimized.

An application of the invention has been described in which there isonly one single input stage and one single conversion device.

The invention also applies to charge conversion in a matrix 100 of imagepixels 120 ₁₁-120 _(LC) arranged in L lines by C columns. For eachcolumn of the matrix 100, the L pixels 120 _(1j)-120 _(Lj) are connectedvia L circuit breakers 44 _(1j)-44 _(Lj) to a column bus 31 _(j) whichhas a parasitic capacitor 40 _(j) relative to the earth. Each of these Cunits 120 ₁₁-120 _(LC), 44 _(1j)-44 _(Lj), 31 _(j) and 40 _(j)constitutes an input stage 12 _(j). The matrix 100 therefore breaks downinto C input stages 12 ₁-12 _(C).

A prior art charge converter 102 is shown in FIG. 8.

The prior art charge converter 102 comprises C charge conversion devices10 ₁-10 _(C). Each of these conversion devices 10 ₁-10 _(C) is connectedto one of the input stages 12 ₁-12 _(C) constituting the matrix ofpixels 100 and is capable of being connected to a single output stage 14_(m) constituted by a column multiplexer of capacitance C_(mux).

The conversion device 10 ₁-10 _(C) includes an amplifier 16 ₁-16 _(C)with inverse feedback via a capacitor 18 ₁-18 _(C) of value C_(fbj) andconnected between the inverting terminal 20 ₁-20 _(C) and the output 22₁-22 _(C) of the amplifier, a first controllable circuit breaker 42 ₁-42_(C) connected in parallel with the capacitor 18 ₁-18 _(C), L secondcontrollable circuit breakers, 44 _(1,1)-44 _(L,C) capable of connectingthe inverting terminal 20 ₁-20 _(C) of the amplifier 16 ₁-16 _(C) toeach of the pixels of the column, and a third controllable circuitbreaker 52 ₁-52 _(C) connected to the output 22 ₁-22 _(C) of theamplifier.

Lastly, a generator 70 _(m) of control signals is also provided tocontrol the first, second and third circuit breakers 42 ₁-42 _(C), 44_(1,1)-44 _(L,C), 52 ₁-52 _(C). To be more specific, the generator 70_(m) controls the first circuit breakers 42 ₁-42 _(C) by means of asignal φ₁, the C second circuit breakers 44 _(1,1)-44 _(L,C) in a singleline of the matrix 100 by means of a signal φ_(2L1)−φ_(2LL)respectively, and each of the third circuit breakers 52 ₁-52 _(C) bymeans of a signal φ_(2C1)−φ_(2CC) respectively.

The operation of the prior art charge converter 102 is shown in FIGS. 9to 17.

As can be seen in FIG. 11, a so-called “readout” frame of the matrix 100comprises a first phase 200 of exposure thereof to radiation, duringwhich charges are generated in the pixels 120 ₁₁-120 _(LC), followed bya phase 202 for the readout of said charges. During the readout phase202, the matrix 100 is read pixel line by pixel line and thus includes Lsuccessive phases for the readout of lines L₁-L_(L).

As can be seen in FIGS. 11 to 17, during a phase comprising theinitialization of the readout of a line of pixels of the matrix 100, theC first circuit breakers 42 ₁-42 _(C) are off in order to discharge thecapacitors 18 ₁-18 _(C) and the second and third circuit breakers 44_(L,1)-44 _(L,C), 52 ₁-52 _(C) are on.

During a second phase comprising the readout of the line of pixels, theC first circuit breakers 42 ₁-42 _(C) are on, the second circuitbreakers 44 _(1,1)-44 _(L,C) associated with the pixels of this line areoff and the third circuit breakers 52 ₁-52 _(C) at output from thecharge conversion devices 10 ₁-10 _(C) are off momentarily in turn sothat charge can be transferred to the column multiplexer 14 _(m).

It will thus be noted that a charge conversion device 10 ₁-10 _(C)implements the sequential conversion of the charges produced by L pixelsof a column of the matrix 100.

However, during the charge conversion, the charge conversion device 10₁-10 _(C) is constantly connected to the column bus 31 ₁-31 _(C) andtherefore to its parasitic capacitor C_(bus1)-C_(busC). In fact, theparasitic capacitance of a column bus is commonly very high, whichtranslates into very significant equivalent capacitance C_(eq). Thispermanent connection to the column bus 31 ₁-31 _(C) thereby very muchlimits the charge-to-voltage conversion establishment time.

A charge converter 302 of the invention, as shown in FIG. 18, alsocomprises C charge conversion devices 60 ₁-60 _(C). These chargeconversion devices 60 ₁-60 _(C) differ from those of the prior art inthat it further comprises fourth controllable circuit breakers 62 ₁-62_(C) arranged between the non-inverting inputs 20 ₁-20 _(C) of theamplifiers 16 ₁-16 _(C) and the column buses 31 ₁-31 _(C) respectively.

The fourth circuit breakers 62 ₁-62 _(C) are controlled by a generator80 _(m) of control signals which differs from the prior art generator 70_(m) in that it further produces a control signal φ_(2BUS) of the fourthcircuit breakers 62 ₁-62 _(C).

The operation of the charge converter 302 of the invention is shown inFIGS. 19 to 28.

This operation differs from that in the prior art shown in FIGS. 9 to17, in that the input of the charge conversion devices 60 ₁-60 _(C) isdisconnected from the column buses 31 ₁-31 _(C) prior to the transfersof voltages resulting from the charge conversions stored at theterminals of the capacitors 18 ₁-18 _(C) to the column multiplexer 14_(m). This therefore makes it possible, during sequential establishmentof the voltages V_(out) in the output stage 14 _(m), to isolate thecharge conversion devices 60 ₁-60 _(C) from their main input parasiticcapacitors C_(bus1)-C_(busC). Thus, as can be seen in FIG. 28, thecircuit breakers 62 ₁-62 _(C) are continually on, except for a timeinterval needed to transfer the charges from the pixels into thecapacitors 18 ₁-18 _(C).

A very significant reduction in the charge conversion deviceestablishment time, together with a correct establishment of the voltageV_(out) when reading out the matrix 100, are thus obtained, as is shownin FIGS. 29 to 34. FIGS. 29 to 34 show, for the readout of a line i ofthe matrix 100, the charge conversion of the pixel of column j. As canbe observed in FIG. 39, the permanent connection of a prior art chargeconversion device to a column bus does not allow the charge in itsentirety to be converted because the establishment time is too high. Onthe other hand, according to the invention, the effect of isolating theconversion device from the column bus when establishing the voltageV_(out) in the output stage is a significant drop in the establishmenttime and a complete conversion of the charge.

For the same reasons as those mentioned previously in relation to thedevice in FIG. 2, the circuit breakers 62 ₁-62 _(C) and the circuitbreakers 52 ₁-52 _(C) are placed as close as possible to the amplifiersconstituted by the operational amplifiers 16 ₁-16 _(C) with inversefeedback via the capacitors 18 ₁-18 _(C).

An embodiment of the invention has been described wherein N chargeconversion devices (10 _(l), 10 _(j), 10 _(N),) convert the charges of amatrix of pixels arranged in L lines by C columns with the specialfeature N=C by dint of having one charge conversion device per column.

Clearly the invention also applies to instances where it is necessary topool the charge conversion devices in respect of a plurality of columns,wherein there is the relationship N=C/M with M representing for examplethe number of columns associated with one charge conversion device.

The invention also applies in instances where a plurality of chargeconversion devices are associated in respect of a single column, whereinthere is the relationship N=C.P with P representing for example thenumber of charge conversion devices per column.

Arrangements and operating modes have been described fornon-differential charge conversion devices.

Clearly, the invention also applies to differential charge conversion bya differential amplifier whereof each output is mounted in inversefeedback on an input by means of a capacitor. A differential chargeconversion device 60 d is shown in FIG. 35. This device 60 d repeats theinverse feedback and circuit breaker structure described in relation toFIG. 2 for each of the inverting and non-inverting inputs of anamplifier 16 d.

The two branches of the amplifier are then controlled synchronouslyaccording to a phase comprising the amplification and storage of thecharges in the inverse feedback capacitors and a phase comprising thetransfer of the stored charges to the output stage described in relationto FIGS. 2 to 4.

By means of the invention, the following advantages are thus obtained:

an appreciable gain in charge conversion speed by means of a reductionin the equivalent capacitances;

increased independence of the charge conversion characteristics inrespect of the input and output stages;

a minor modification of the structure of prior art charge conversiondevices;

a possibility of increasing the charge conversion frequency, by reducingfor example the readout time of a matrix of pixels;

a possibility of increasing the charge-to-voltage conversion gain whileretaining the same conversion time; and

a possibility of reducing the consumption and/or the surface of theamplifiers by reducing the transconductance g_(m).

1. A method for controlling a device for converting charge into voltagecomprising an amplifier and at least one capacitor mounted in inversefeedback between an input and an output of said amplifier, whereby saidamplifier can be connected between at least one input stage, to receiveat least one charge therefrom, and at least one output stage to delivervoltage thereto, said voltage being representative of the chargereceived, said method comprising at least one phase of conversion of thecharge received at the input, wherein the conversion phase comprises atleast: one first sub-phase during which the amplifier is connected tothe input stage and the amplifier is disconnected from the output stage;followed by a second sub-phase during which the amplifier isdisconnected from the input stage and the amplifier is connected to theoutput stage, and wherein the input stage is of the MEMS or NEMS typeand comprises a time-variable capacitor, the input stage producing acharge as a function of said variable capacitor.
 2. The control methodas claimed in claim 1, wherein the durations of the two sub-phases aresubstantially identical.
 3. The control method as claimed in claim 1,wherein the durations of the two sub-phases are regulated as a functionof the time constants of the units formed, on the one hand, by theconversion device connected to the input stage, and on the other hand,by the conversion device connected to the output stage.
 4. The controlmethod as claimed in claim 1, wherein the or each amplifier is anoperational amplifier, whereof the inverting input and the output aremounted in inverse feedback via a capacitor.
 5. The control method asclaimed in claim 1, wherein the or each amplifier is a differentialamplifier whereof each input is mounted in inverse feedback with anoutput of the amplifier by means of a capacitor.
 6. The control methodas claimed in claim 1, wherein the location of the connection anddisconnection of the amplifier at the input stage and/or the location ofthe connection and disconnection of the amplifier at the output stage isplaced as close as possible to the amplifier.
 7. A system for convertingcharge comprising: a device for converting charge into voltagecomprising an amplifier and at least one capacitor mounted in inversefeedback between an input and an output of said amplifier, whereby saidamplifier can be connected between at least one input stage, to receiveat least one charge therefrom, and at least one output stage to delivervoltage thereto, said voltage being representative of the chargereceived; a first controllable element capable of disconnecting theamplifier from the input stage; and a control device capable ofcontrolling the first controllable element according to a first phase ofconversion of a charge received from the input stage by the amplifier,wherein it further comprises a second controllable element capable ofdisconnecting the amplifier from the output stage, the control devicebeing capable, during the conversion phase, at least of: controlling thefirst element in order to connect the amplifier to the input stage, andcontrolling the second element in order to disconnect the amplifier fromthe output stage; and then, controlling the first element in order todisconnect the amplifier from the input stage and controlling the secondelement in order to connect the amplifier to the output stage; andwherein the input stage is of the MEMS or NEMS type and comprises atime-variable capacitor, the input stage producing a charge as afunction of said variable capacitor.
 8. The system for converting chargeas claimed in claim 7, wherein the first controllable element and/or thesecond controllable element is placed as close as possible to theamplifier.